Materials typically exhibit a variety of different failure mechanisms, depending upon the type of material, how it is manufactured, and the stresses present upon the material for its intended use. There are rigid and brittle materials going all the way toward gummy and soft materials, especially related to metals, like cast iron would be rigid and brittle and aluminum would be soft and gummy, with a typical carbon steel being somewhere between these two extremes being softer and more pliable than cast iron, however, having more rigidity and strength than aluminum. This is why carbon steel is a popular all around material to use as it has the flexibility for many different applications, especially for example where you would have heavy bearing and shear loads upon the material, as in the case of a thread. Wherein on a thread you would have a high bearing load from the metal to metal face contact of the thread flank faces pressing as against one another with a high force coupled with a high shear stress component occurring between the mating threads that is parallel to the radial axis of the threads or more commonly known at the thread “pitch line” which is the theoretical axis at somewhat of a mid-point as between the mated threads running parallel to the thread radial axis.
Thus an ideal material for threads is first a hard faced material that would do well under the high bearing load, such as to resist inter-granular adhesion as between the thread flanks, which can make threaded parts virtually impossible to disassemble. Further, another aspect of the ideal thread material would be to have good shear resistance which would means in the thread case for the material to be somewhat soft and flexible to “give” when under thread load so that a thread has a gradual “tightening” feel as the shear deflection gradually increases. This would be as opposed to a very rigid material that would deflect little from shear stress and then suddenly fail in a shear fracture, meaning that as an individual would tighten the thread is would suddenly “strip” and be ruined without much warning when tightening the thread, which would be undesirable. However, on the other hand if the material were too soft and gummy, the thread in shear would fail also without warning, wherein the individual would be tightening the thread and without much rotational resistance, the thread would “strip” easily.
Typically, the thread shear stress and thread flank bearing load are controlled by the rotational torque applied to the thread, however, in the real world this is highly inaccurate for one predominating reason being that controlling of the torque assumes a constant coefficient of friction as between the thread flanks as they are sliding as against one another, wherein this assumption of the friction factor is often highly inaccurate and can change dramatically depending upon the bearing load, surface finish of the thread flanks, and cleanliness/lubrication of the thread flank surfaces. The torque control problems also apply to the individual who tightens the threads by “feel” which is the same as torque control and thus has all of the same aforementioned problems. Ideally the way to properly tighten a thread as in critical applications such as a pressure vessel flange, is to hydraulically pre-stretch the threaded stud to the proper preload in tensile stress and ten to hand snug the threaded nut upon the stud, then relieving the hydraulic “preload stretch” wherein the stud will pull the threaded nut into axial tightness, this method completely eliminates the variability of the previously mentioned problem of thread flank to flank friction as it is not relied upon at all.
Unfortunately, the hydraulic method is not possible without a lot of free volumetric space around the assembled thread, plus it is quite costly and really only justified for safety critical thread retaining applications, such as a pressure vessel, wherein failure of the thread unexpectedly would result in a high degree of danger. As ideally the thread should see its maximum stress during assembly and a lower stress during use, thus this would eliminate an unexpected failure of the thread in later use, i.e. if it does not strip during assembly it never will after that.
However, in the real world the best material are not always used for economic reasons (too soft or too hard), high inaccuracies exist due to the torque issue previously discussed resulting in overloaded threads further resulting in deformed, damaged, or striped threads being an all too common occurrence. The well recognized problem is in the difficulty in repairing the threads, as the threads may be just a portion of a much larger machine, making it difficult to remove and isolate the failed threaded area for repair, plus there is always the consideration of what the failed thread mates with, as the thread repair usually requires restoration back to its original size before it failed. This restoration requires the adding of material by welding, inset, or otherwise, which can be difficult given that the failed thread may not be easy to isolate for the adding of material.
Further, due to carbon buildup within the combustion chamber from the by-products of fuel combustion in conjunction with the high pressures and temperatures present in the combustion chamber causes carbon deposits both in the thread area and the extended arc producing tip that protrudes into the combustion chamber through the engine head. Over time these carbon deposits essentially “weld” the spark plug to the engine head especially in the thread and/or extended arc producing tip areas, and when it comes time to remove and replace the spark plug and rotational torque is applied to the spark plug to unscrew it, the spark plug essentially yields (breaks) somewhere between the thread/extended arc producing tip that is welded to the engine head and the wrench hex attachment, causing substantial repair time and cost. Thus the repair typically results in the engine head requiring extensive rework in drilling out the broken extended arc producing tip/threads and re-threading the engine head for a new spark plug, plus the difficult job of cleaning out the combustion chamber of debris, or having to remove the engine head from the engine to gain access to the broken extended arc producing tip. This repair or portions of it recognized in the prior art with the following examples given.
Starting with U.S. Pat. No. 6,439,817 to Reed disclosed is an insert retention mechanism. The insert in Reed is a substantially cylindrical construct having an exterior thread which meshes with the newly threaded bore of the casting and an interior bore having threads complementary to the dimensions of the preexisting fastener previously residing within the old bore. In this way, in Reed the same sized fastener or spark plug that was installed originally within the metal casting can be used after the repair. Besides fasteners and spark plugs, the insert in Reed also finds utility, inter alia, for repairing hydraulic fitting threads, pipe threads and as a blind hole insert. Moreover, in Reed the instant invention addresses and resolves any problems associated with an attempt to subsequently remove the fastener or spark plug after the repair. In some situations, typically harsh operating environments involving corrosion or galvanic attraction between the various components of a system, the mating area between the threads of the fastener or spark plug can become seized to the insert. When this occurs, an attempt to remove the fastener or spark plug can sometimes cause rotation of the insert in conjunction with the fastener or spark plug, thwarting removal of the fastener or the spark plug alone.
Thus, the solution in Reed preferably includes the utilization of both specially formed threads and a shoulder on the insert which is adapted to provide a cylindrical bore strategically located to vertically align with the meshing exterior threads of the insert and the threads formed in the bore of the material being worked on. A top surface of the insert's shoulder in Reed includes a cylindrical bore. After the insert has been placed within the material to be repaired in Reed, a hole may be drilled extending the cylindrical bore into the juncture of the exterior threads of the insert and the threads of the bore in the material. Finally, in Reed, a cylindrical pin is driven into the cylindrical bore through the shoulder and into the drilled area of the exterior threads of the insert and the threads of the bore of the material so that the insert will no longer readily move with respect to the material because the flight of the threads of the insert on an exterior surface thereof will be opposed by the placement of the cylindrical pin and its retention by the threads of the bore of the material. Where the insert in Reed already includes a vertical channel defining a thread gap aligned with the cylindrical bore of the insert's shoulder, the drilling step is not mandatory. In this case, for Reed driving the cylindrical pin will actually improve insert retention because the threads in the bore contacted by the pin distort and therefore enhance retention of the insert in the bore, see Column 2, lines 26-67, and Column 3, lines 1-5. Note that in Reed, the pin driving into the threads is common in this art area to retain the threaded insert into the larger rethreaded base material, however, it is not optimum at all as the pin deforming the base material threads causes stress risers due to sharp edges that can lead to base material cracking, thereby causing the thread repair to ultimately cause more damage to the base material.
Continuing in the threaded insert prior art area in looking at U.S. Pat. No. 5,411,357 to Viscio, et al. disclosed is a screw thread locking insert for locking a threaded insert into a prepared hole in a parent material. The device in Viscio et al., includes a locating portion, a locking portion and a gripping portion which is removed upon installation. The locating portion in Viscio et al., comprises a finger which is positioned in a preformed slot in the external threads of the threaded insert. The locking portion in Viscio et al., which extends outwardly from the locating portion, is driven across the corresponding threads of the parent material to shear and distort the threads and lock the insert in place. The gripping portion in Viscio et al., which extends outwardly from the locking portion is used by the installer to position the device during installation and is then broken off. Note also that as in Reed, Viscio et al., has the same undesirable issue relating to the drive pin being driven into the base material threads.
Further in the threaded insert prior art, in looking at U.S. Pat. No. 4,325,665 to Jukes disclosed a self-locking insert having a generally tubular shape with substantial portions of the exterior and the interior being threaded. The interior in Jukes includes a portion which does not have complete threads. Positioned within the exterior thread in Jukes outwardly from the incomplete threads of the interior are one or more locking plugs. When installing the insert in Jukes, a threaded insert driver is threaded into the interior of the insert until it engages the incompletely threaded portion of the interior. The insert in Jukes is then threaded into a tapped hole in a base material until a flanged or outwardly flared head on the exterior of the insert engages the base material. The insert driver in Jukes is then forcibly rotated further to complete the threads in the interior of the insert which creates a force outward against the walls of the insert. This force in Jukes urges the locking plugs outward more easily than the portion of the insert surrounding the plugs so that the plugs engage the walls of the tapped hole and securely lock the insert in place. Preferably in Jukes, the apertures in which the plugs are positioned, extend at an angle with respect to a radial line of the insert, when a spherical plug is used, wherein this cams a spherical plug to engage more tightly into the walls of the tapped hole, when torque is applied to attempt to remove the insert, see Column 2, lines 23-48. Thus in Jukes, with the outwardly biased thread plugs an attempt is made to minimize the negative stress riser effect from the previously discussed pins to accomplish the same function of preventing reverse rotation of the threaded insert.
Continuing in the prior art, also in looking at U.S. Pat. No. 6,668,784 to Sellers, et al. disclosed a thread insert and method to replace the damaged threads and tapered seat in a spark plug bore of an internal combustion engine that allows for the continued use of the original factory specified spark plugs where the original threads in the spark plug bore have been damaged by stripping or cross threading. The thread insert's inner bore in Sellers, et al. is designed to replace the original threads and tapered seat in the cylinder head. The thread insert in Sellers, et al. may be adapted to fit any internal combustion engine using tapered seat spark plugs, and is particularly useful in deep spark plug bores with limited access as found in the aluminum heads of Ford Motor Company modular engines. The insert in Sellers, et al. includes a flange head that determines how far into the head the insert can extend and a recess below the flange to collect any bonding agent that may be squeezed from the threads during installation of the insert. Special tools in Sellers, et al. make the installation of the insert easy and accurate. Note that also Sellers, et al. recognizes the drive pin problems in causing stress risers in the threads in the base material by Sellers, et al. using the bonding agent in the chamber to lock the insert into the base material oversized new threaded hole.
Moving ahead in the prior art for threaded inserts, looking at U.S. Pat. No. 4,730,968 to Diperstein, et al. disclosed a self-tapping, self-aligning thread repair insert. The insert in Diperstein, et al. is an annular sleeve having a threaded interior surface, a partially threaded exterior surface, and an opening in the form of a slot. The exterior surface in Diperstein, et al. has a tapered portion between a straight threaded portion and a straight thread-free portion. The thread-free portion in Diperstein, et al. and the opening are adjacent an end of the sleeve. The thread-free portion surface in Diperstein, et al. is free of threads for a distance of at least 1.5 thread widths from the end of the sleeve, see Column 1, lines 35-45. Diperstein, et al. uses a self tapping threaded insert which can save the use of some additional tooling that most of the other prior art requires, however, the strength of the insert can be compromised due to the self tapping slits, see FIGS. 5 and 6, wherein the thread length is less than the thread diameter, meaning that the threads are weaker than the bolt, further as previously mentioned having to tap new threads for a larger hole in the base material leaves metal chips that come from the cutting of the new larger threads. The problem of these metal chips is that say in an automotive engine cylinder head they would fall into the cylinder and cause gouging of the finely honed cylinder sidewalls that will lead to excessive engine wear and damage, the way out of this would be to remove the head from the engine, being a difficult and time consuming task.
What is needed is a spark plug removal tool apparatus that helps to remove the spark plug from the engine head without destroying the spark plug in the first place to eliminate the need for the expensive and time consuming repair of removing the damaged spark plug from the engine head as previously described.